What is it?

One of these were fitted to most MGB Roadsters and GTs when they left the factory but that was 40+ years ago. Most have gone missing and most likely removed and discarded by garages because they were a nuisance and awkward to refit.
It is a cover that fits over the starter motor that protects the terminals from their very exposed position to the weather.
The secret to fitting it is to put it in a container of very hot water for a few minutes prior to fitting …….but it still has to be done from underneath the car.